Shigeki Miyaji

The possibility of a detonative shell flash on a neutron star and a gamma‐ray burst

Recent observations of gamma‐ray bursts (e.g., x‐ray precursor, x‐ray tail, and cyclotron lines observed by Ginga) strongly suggest that the gamma‐ray burst source is associated with a magnetized neutron star. As an energy source model the shell flash is one of the most studied and its detonative wave has been suggested as an energy conversion mechanism from thermal energy to gamma‐ray photons. Here we present results of numerical simulations of shell flashes of a mass‐accreting neutron star. Models with mass accretion rate dM/dt≳10−13 M⊙/yr do not trigger a detonative wave. A model with dM/dt≳10−15 M⊙/yr is marginal and its recurrence time is ∼106 yr, so that the detonative wave model is statistically inconsistent. An alternative mechanism of energy conversion would be an Alfven wave generated in a convective region of a deflagration wave. In this case, the Alfven wave is generated by a vibration of the magnetic field in the convective region where the thermal pressure is larger than the magnetic pressur...

Performance of Vector/Parallel Orientated Hydrodynamic Code

We have developed a hydrodynamic 3 dimensional AMR code which is suitable for vectorization and parallelization based on Fully Threaded Tree method. For the case of 3D FTT-AMR simulation, the number of higher resolution cells is eight times of that of mother low resolution cells. Therefore if the region where the highest resolution is required has certain volume, it is better to use single time step for every levels of resolution because we can avoid artificial flow at the surface of resolution interface. This level-independent time step also makes possible to simplify the evaluation of refinement indicator. As a result, we can overcome the demerit of consuming CPU time by computing at level-independent time step less than CFL limit, by boosting vector acceleration ratio.

Magnetohydrodynamic Simulations of the Wiggle Instability in Spiral Galaxies

We studied the stability of galactic spiral shocks through two dimensional global magnetohydrodynamic simulations. Recently, Wada & Koda (2003) showed, using global hydrodynamic simulations, that galactic gas flows behind a spiral shock becomes unstable against a perturbation parallel to the shock front and form spur‐like density structures. They attributed the origin of this wiggle instability to the Kelvin‐Helmholtz (K‐H) instability triggered by the acceleration of the gas behind the shock. We carried out global simulations including galactic magnetic fields. The initial magnetic field is assumed to be either uniform or purely toroidal. We found that although the magnetic field reduces the growth rate of the K‐H instability, wiggle instability develops even in galaxies with μG magnetic fields. We also present the results of local simulations to demonstrate the dependence of the growth rate of the instability with the wavelength. The interval of spurs is determined by the most unstable wavelength of the...

Performance of Adaptive Mesh Refinement Scheme for Hydrodynamics on Simulations of Expanding Supernova Envelope

We present results of performance measurement of our astrophysical fluid dynamics code with Adaptive Mesh Refinement (AMR) scheme. As an example of its application for astrophysical phenomena, we show the results of 3D simulation of Rayleigh-Taylor instability in expanding supernova envelope and the possibility of reconciliation of the present model with observations. The efficiency of memory and CPU-time savings is discussed and measured with this simulation. Its result shows that our code has the ability to simulate phenomena with wide dynamic ranges even in limited computing resources.

Global Magneto-Hydrodynamic Simulations of Differentially Rotating Accretion Disk by Astrophysical Rotational Plasma Simulator

We present numerical results of three-dimensional global magneto-hydrodynamic (MHD) simulations achieved on Astrophysical Rotating Plasma Simulator (ARPS) developed at Chiba University. We simulate the time evolution of differentially rotating disks by using a parallelized three-dimensional MHD code. Typical number of grid points is (Nr,NΦ,Nz) = (200, 64, 240) in a cylindrical coordinate system. We found that when the initial magnetic field is toroidal and relatively strong, the system approaches a quasi-steady state with β = Pgas/Pmag ∼ 5. When the disk is threaded by vertical magnetic fields, magnetically driven collimated jet emanates from the surface of the disk. Fully vector-parallelized global simulations with ARPS enable us to study non-local effects such as magnetic pinch, saturation of nonlinear growth of instability, and deformation of the global structure.